Miniature radio frequency transmitters have been designed that are usually worn on the wrist, ankle or elsewhere by a person who, for various reasons, needs to be prevented from entering or leaving certain surroundings. Examples of such persons are medical patients who are suffering from Alzheimer's disease who have a tendency to become confused and wander away from safe surroundings, inmates who are serving sentences either in institutions or half-way houses or in their own homes, and newly born infants in hospitals who may be stolen by distressed adults or professionals. A transmitter attached to such a person, operating as a beacon, triggers a radio frequency receiver when it is adjacent a building exit, and warns of the exiting of the transmitter and therefore of the person.
Such transmitters are usually housed in a wrist watch type of case, with a strap around the wrist or ankle. A wire in the strap is connected at two points through the housing, one of those points to the transmitter. A signal such as a direct current is transmitted through the wire, and is received by a receiver also contained in the housing, whereby the retention of the strap in place can be verified.
If it is desired to remove the beacon without authorization, one of the techniques is to cut the strap. This interrupts the signal, and as a result the beacon transmits a special alarm signal.
However a resourceful person could find the means to short circuit the wire at the two points of entry to the housing, or at some other suitable point, thus maintaining the continuity of the circuit. It has therefore been found that such straps are unreliable, since they can be defeated.
Since the beacon in the housing is worn at all times, including when bathing or showering, the transmitter must be housed in a waterproof container. This creates technical problems in bringing electrical contacts for connection to the external wire out of the housing. The presence of water, soaps, sweat and body oils necessitates the connect point to the embedded wire should be gas and water tight to prevent corrosion and consequent device malfunctioning.
Further, it has been found that the presence of the wire in the strap has undesirable effects on the radiation patterns of the radio frequency transmitter of the beacon.